In drilling wells in hydrocarbon-producing formations, drilling fluid circulating in the drill string acts to dissipate heat from the drill bit, carry cuttings from the wellbore to the surface, transmit information about drilling conditions, and so forth. For reasons including cost reduction and environmental protection, it is desirable to treat used drilling fluid to separate the solids from liquid and to clarify the liquid to certain given standards. Drilling fluid treatment apparatus known in the art are often too unwieldy to be conveniently and economically used at the drilling site, while other known treatment processes often produce water that must be treated further to be sufficiently devoid of drilling impurities (for example, to satisfy environmental standards) and/or do not separate out sufficient amounts of clarified liquid thereby producing more dilute than necessary amounts of waste sludge.